Level measurements by Guided Wave Radar (GWR) measurements are performed by sending a microwave sub-nanosecond pulse along a guiding probe and measuring the time elapsed until the echo returns from the surface a product material in a storage tank (or vessel). By the same principle, interfaces between product media (or materials) with different dielectric constants in storage tanks can also be measured. The GWR level transmitters can have a variety of different probe types, or can be attached in many different ways (e.g., threaded or flange process connection) to either metallic or non-metallic storage tanks.
For GWR level transmitters mounted by threaded process connection to non-metallic vessels, the conventional solution involves threading a feed-through that is connected to a transmission line connector such as a coaxial cable on a transceiver side of the electronic block into a coupling device comprising a planar metal sheet or planar metal foil sealed to the top surface of the storage tank. For example, a threaded process connection including a feed-through coupled to a coupling device comprising a planar metal sheet or planar metal foil may be made over an aperture in a concrete storage tank (also referred to as a concrete “silo”) for GWR measurement of the level of solid products in the tank, such as wheat, plastic pellets or cement.
The feed-through together with the planar metal foil coupling device together provide a so-called “launcher” which has not only a mechanical function of process sealing the storage tank's contents from the outside ambient, but also an electromagnetic function of converting the Transversal Electric and Magnetic (TEM) mode of microwave propagation inside the dielectric of the transmission line connector (e.g., coaxial cable) received by the feed-through of the launcher into the Transversal Magnetic (TM) mode outside the launcher (i.e. along the probe). The TM mode is specific to microwave propagation along the probe. This electromagnetic mode conversion from TEM mode to TM mode is made possible by the feed-through electrically connecting the outer conductor sleeve of the transmission line connector (e.g., coaxial cable) to the metal foil coupling device.